1. Field of the Invention
This invention relates to circuitry for use in motor drivers, and more particularly to improvements in slew rate controlling circuits for use in drivers for polyphase dc motors or the like.
2. Technical Background
In the past, the slew rate of the upper driver in polyphase dc motor driver circuits was controlled in an approximate or rough manner by an R-C circuit connected to the gate of the driver transistor. The R-C circuit was designed to discharge the gate and to provide a slew rate upon the gate that approximately equaled the particular slew rate of the high side of the driver to which the drive transistor was connected. It can therefore be seen that the charging and discharging of the gate of the driver transistor was at a limited, fixed rate, depending upon the particular R-C constant selected.
To "optimize" the slew rate, typically the resistor, was trimmed for the particular application. Thus, each application required specific adjustment of the R-C circuit. This was dependent upon many factors, some of which were very difficult to control such as the parasitic capacitances of the driver transistor and so on. Thus, the particular design of the R-C circuit that was employed within a particular driver circuit was dependent upon the characteristics of the particular driver transistor that was used.
Two major problems exist in this type of compensation. The first problem is that the slew rate is not very well controlled. This is a result of the non-linear characteristics of the intrinsic capacitors of the driver transistor. The second problem that is encountered is that if the driver transistor were used in usual fashion as a switch, and is heavily turned on, typically the gate voltage is much higher than the source voltage, sometimes by as much as ten volts or more. To turn off the transistor, the over voltage first needed to be discharged before the turnoff of the transistor could be effected. After that the slew rate circuit would become effective to produce the desired turn off slew rate.
Since the R-C circuit is essentially custom designed for the slew rate, it also controls the over voltage discharge at its particular discharge rate. This results in a delay before the slewing characteristics desired are initiated. That is, nothing in the output occurs until the gate voltage discharges to a value close to the threshold for the particular transistor. The result is a delay from the turn off command that is dependent upon the slew rate circuit design.